Tissue
distribution

Structure

To function, CD8 forms a dimer, consisting of a pair of CD8
chains. The most common form of CD8 is composed of a CD8-α and
CD8-β chain, both members of the immunoglobulin superfamily
with an immunoglobulin variable (IgV)-like
extracellular domain connected to the membrane by a thin stalk, and
an intracellular tail. Less-common homodimers of the CD8-α chain
are also expressed on some cells. The molecular weight of CD8 is
about 13,463.2 Da. Structure of the CD8 molecule was determined by
Leahy, D.J., Axel, R., and Hendrickson, W.A. by X-ray Diffraction
at a 2.6A resolution.[1]
The structure was determined to have an immunoglobulin-like
beta-sandwich folding and 114 amino acid residues. 2% of the
protein is wound into α-helices and 46% into β-sheets, with the
remaining 52% of the molecules remaining in the loop portions.

Function

The extracellular IgV-like domain of CD8-α interacts with to the
α3 portion of the Class I MHC molecule.[3] This
affinity keeps the T cell receptor of the cytotoxic T cell
and the target cell bound closely together during antigen-specific
activation. Cytotoxic T cells with CD8 surface protein are called
CD8+ T cells. The main recognition site is a flexible loop at the
a3 domain of an MHC molecule. This was discovered by doing
mutational analyses. The flexible a3 domain is located between
residues 223 and 229 in the genome.[1]